Extended Memory - Overview

Overview

On x86-based PCs, extended memory is only available with an Intel 80286 processor or higher. Only these chips can address more than 1 megabyte of RAM. The earlier 8086/8088 processors can make use of more than 1 MB of RAM, if one employs special hardware to make selectable parts of it appear at addresses below 1 MB.

On a 286 or better PC equipped with more than 640 kB of RAM, the additional memory would generally be re-mapped above the 1 MB boundary, since the IBM PC architecture reserves addresses between 640 kB and 1 MB for system ROM and peripherals.

Extended memory is not accessible in real mode (except for a small portion called the high memory area). Only applications executing in protected mode can use extended memory directly. A supervising protected-mode operating system such as Microsoft Windows manages application programs access to memory. The processor makes this memory available through the Global Descriptor Table (GDT) and one or more Local Descriptor Tables (LDTs). The memory is "protected" in the sense that memory segments assigned a local descriptor cannot be accessed by another program because that program uses a different LDT, and memory segments assigned a global descriptor can have their access rights restricted, causing a processor exception (e.g., a general protection fault or GPF) on violation. This prevents programs running in protected mode from interfering with each other's memory.

A protected-mode operating system such as Microsoft Windows can also run real-mode programs and provide expanded memory to them. The DOS Protected Mode Interface (DPMI) is Microsoft's prescribed method for an MS-DOS program to access extended memory under a multitasking environment.